The following correction was printed in the Guardian's Corrections and clarifications column, Saturday 14 November 2009

In the report below about sex trafficking we referred to the United Kingdom Human Trafficking Centre as "the police Human Trafficking Centre". The UKHTC describes itself as "a multi-agency centre" and says that it is "police led". Its partners include two non-governmental organisations, HM Revenue & Customs, the Crown Prosecution Service, the Serious Organised Crime Agency and the UK Border Agency. We referred to Grahame Maxwell as the head of the UKHTC; his title is programme director.

The UK's biggest ever investigation of sex trafficking failed to find a single person who had forced anybody into prostitution in spite of hundreds of raids on sex workers in a six-month campaign by government departments, specialist agencies and every police force in the country.

The failure has been disclosed by a Guardian investigation which also suggests that the scale of and nature of sex trafficking into the UK has been exaggerated by politicians and media.

Current and former ministers have claimed that thousands of women have been imported into the UK and forced to work as sex slaves, but most of these statements were either based on distortions of quoted sources or fabrications without any source at all.

While some prosecutions have been made, the Guardian investigation suggests the number of people who have been brought into the UK and forced against their will into prostitution is much smaller than claimed; and that the problem of trafficking is one of a cluster of factors which expose sex workers to coercion and exploitation.

Acting on the distorted information, the government has produced a bill, now moving through its final parliamentary phase, which itself has provoked an outcry from sex workers who complain that, instead of protecting them, it will expose them to extra danger.

When police in July last year announced the results of Operation Pentameter Two, Jacqui Smith, then home secretary, hailed it as "a great success". Its operational head, Tim Brain, said it had seriously disrupted organised crime networks responsible for human trafficking. "The figures show how successful we have been in achieving our goals," he said.

Those figures credited Pentameter with "arresting 528 criminals associated with one of the worst crimes threatening our society". But an internal police analysis of Pentameter, obtained by the Guardian after a lengthy legal struggle, paints a very different picture.

The analysis, produced by the police Human Trafficking Centre in Sheffield and marked "restricted", suggests there was a striking shortage of sex traffickers to be found in spite of six months of effort by all 55 police forces in England, Wales, Scotland and Northern Ireland together with the UK Border Agency, the Serious and Organised Crime Agency, the Foreign Office, the Northern Ireland Office, the Scottish government, the Crown Prosecution Service and various NGOs in what was trumpeted as "the largest ever police crackdown on human trafficking".

The analysis reveals that 10 of the 55 police forces never found anyone to arrest. And 122 of the 528 arrests announced by police never happened: they were wrongly recorded either through honest bureaucratic error or apparent deceit by forces trying to chalk up arrests which they had not made. Among the 406 real arrests, more than half of those arrested (230) were women, and most were never implicated in trafficking at all.

Of the 406 real arrests, 153 had been released weeks before the police announced the success of the operation: 106 of them without any charge at all and 47 after being cautioned for minor offences. Most of the remaining 253 were not accused of trafficking: 73 were charged with immigration breaches; 76 were eventually convicted of non-trafficking offences involving drugs, driving or management of a brothel; others died, absconded or disappeared off police records.

Although police described the operation as "the culmination of months of planning and intelligence-gathering from all those stakeholders involved", the reality was that, during six months of national effort, they found only 96 people to arrest for trafficking, of whom 67 were charged.

Forty-seven of those never made it to court.

Only 22 people were finally prosecuted for trafficking, including two women who had originally been "rescued" as supposed victims. Seven of them were acquitted. The end result was that, after raiding 822 brothels, flats and massage parlours all over the UK, Pentameter finally convicted of trafficking a grand total of only 15 men and women.

Police claimed that Pentameter used the international definition of sex trafficking contained in the UN's Palermo protocol, which involves the use of coercion or deceit to transport an unwilling man or woman into prostitution. But, in reality, Pentameter used a very different definition, from the UK's 2003 Sexual Offences Act, which makes it an offence to transport a man or woman into prostitution even if this involves assisting a willing sex worker.

Internal police documents reveal that 10 of Pentameter's 15 convictions were of men and women who were jailed on the basis that there was no evidence of their coercing the prostitutes they had worked with. There were just five men who were convicted of importing women and forcing them to work as prostitutes. These genuinely were traffickers, but none of them was detected by Pentameter, although its investigations are still continuing.

Two of them — Zhen Xu and Fei Zhang — had been in custody since March 2007, a clear seven months before Pentameter started work in October 2007.

The other three, Ali Arslan, Edward Facuna and Roman Pacan, were arrested and charged as a result of an operation which began when a female victim went to police in April 2006, well over a year before Pentameter Two began, although the arrests were made while Pentameter was running.

The head of the UK Human Trafficking Centre, Grahame Maxwell, who is chief constable of North Yorkshire, acknowledged the importance of the figures: "The facts speak for themselves. I'm not trying to argue with them in any shape or form," he said.

He said he had commissioned fresh research from regional intelligence units to try to get a clearer picture of the scale of sex trafficking. "What we're trying to do is to get it gently back to some reality here," he said.

"It's not where you go down on every street corner in every street in Britain, and there's a trafficked individual.

"There are more people trafficked for labour exploitation than there are for sexual exploitation. We need to redress the balance here. People just seem to grab figures from the air."

Groups who work with trafficked women declined to comment on the figures from the Pentameter Two police operation but said that the problem of trafficking was real.

Ruth Breslin, research and development manager for Eaves which runs the Poppy project for victims of trafficking, said: "I don't know the ins and outs of the police operation. It is incredibly difficult to establish prevalence because of the undercover and potentially criminal nature of trafficking and also, we feel, because of the fear that many women have in coming forward."

The internal analysis of Pentameter notes that some records could not be found and Brain, who is chief constable of Gloucestershire, argued that some genuine traffickers may have been charged with non-trafficking offences because of the availability of evidence but he conceded that he could point to no case where this had happened.

He said the Sexual Offences Act was "not user friendly" although he said he could not recall whether he had pointed this out to government since the end of Pentameter Two.

Parliament is in the final stages of passing the policing and crime bill which contains a proposal to clamp down on trafficking by penalising any man who has sex with a woman who is "controlled for gain" even if the man is genuinely ignorant of the control. Although the definition of "controlled" has been tightened, sex workers' groups complain that the clause will encourage women to prove that they are not being controlled by working alone on the streets or in a flat without a maid, thus making them more vulnerable to attack.

There are also fears that if the new legislation deters a significant proportion of customers, prostitutes will be pressurised to have sex without condoms in order to bring them back.